Diversity and structure of subtidal rock walls in the Salish Sea: the roles of grazing, oceanography, and long-term change

Abstract

The assembly and structure of communities is dictated by a number of ecological processes at a variety of spatial scales. Here I present an `ecology of places', to emphasize the need for both intensive, small-scale experimentation and the larger-scale context from which to interpret the importance of less easily manipulated processes. Specifically, I investigated the roles of local-scale grazing, mesoscale oceanography, and decadal-scale variation on the diversity and structure of benthic communities on subtidal rock walls in the Salish Sea (inland waters of Washington State). In the first chapter, I tested the hypothesis that consumers mediate natural variation in the relationship between prey richness and resource use on epilithic communities. Ecological theory and previous experimental work in dock communities predicted that resource (space) use is a negative, linear function of the number of sessile species present. Contrary to these predictions, a three-month field experiment demonstrated that the relationship between prey richness and resource use was dependent on urchin density, because urchins control the structure of this community by grazing spatially dominant clonal ascidians and facilitating smaller consumers. Following this work, I tested the effects of consumer identity and the predictions of a structural equation model I had developed in chapter one. Namely, do urchins exert indirect effects by facilitating other consumers? In a year-long factorial field experiment, I reduced the densities of urchins and chitons and discovered that the removal of both consumers resulted in unexpected, non-additive changes in community structure. These results suggest that facilitation and redundancy among consumers contribute to the resiliency of species-depauperate habitat dominated by encrusting algae, even if urchins are transient and do not persist indefinitely. To place the local-scale experiments into a broader geographic context, I quantified the effect of mesoscale (10 - 100km) oceanographic variation on the diversity of epilithic communities in my third dissertation chapter. I used a hierarchical sampling design to survey 18 sites, nested within five distinct oceanographic seascapes in Washington. The most striking variation in diversity and composition was observed between seascapes with high and low water retention. Three abiotic correlates of water retention (sediment cover, mass flux, temperature) support the qualitative generalization that waterways and inlets represent distinct physical environments, and consequently harbor unique subtidal biota. Larval delivery and post-settlement mortality are likely to be important mechanisms related to the covarying effects of reduced water flow, sedimentation, and light limitation in high-retention sounds and fjords. Long-term datasets provide a baseline for evaluating temporal variation in biodiversity and are critical for distinguishing between natural and anthropogenic mechanisms of change. My last dissertation chapter tested the hypothesis that the diversity and composition of contemporary (2006-2011) epilithic communities on subtidal rock walls in the San Juan Islands, WA, USA, have changed over thirty years. Despite changes in seawater temperature and chemistry, univariate and multivariate analyses suggest limited differences between historic and modern communities. Historic communities were more even, and characterized by a high percent cover of available space, suggestive of urchin grazing. Despite the initiation of urchin no-take restrictions in 1984, our data indicate that contemporary urchin densities are lower than urchin densities in the 1970's. Declines in biological disturbance (i.e., urchin grazing) will accentuate the naturally low physical disturbance levels on vertical surfaces in subtidal habitats. Although rock walls serve as natural refuges for many invertebrates, a lack of disturbance may allow `weedy' species to dominate and reduce the local diversity of these subtidal communities.